atmospheric density or distance cue, or atmospheric depth or smufatto as they called it in italy. but its basically what you said. the biggest contrast areas are the ones that are closer to the viewers.
my bigest issue with the landscape mockup you've made, is the river and cliff sides. since there's no indication of a cut, or terrain changing, I allways looked at it and it seamed like a buch of tiles that weren't done yet. that you had not finished. you should add some sugestion of an edge or just some haniging grass to help cut into the river. otherwise its a really weird to see such perfectly chopped earth.